Internal combustion engines used for both mobile and stationary applications are subject to strict emission limits. Approaches to reducing emissions include improved in-cylinder combustion designs or fuel modifications, but these improvements have fallen short of meeting emissions limits. Other approaches involve the use of exhaust aftertreatment devices, which have achieved significant emissions reductions.
For diesel engines, particulate matter (PM) emissions are regulated in the United States and other developed nations. However, for spark-ignited engines, currently there are no regulations that address PM emissions. Spark-ignited engines may be either gasoline or alternative fueled engines, the latter including engines fueled by compressed natural gas (CNG), liquefied natural gas (LNG), liquefied petroleum gas (LPG), ethanol, methanol, gas-to-liquid fuel, and coal-to-liquid fuel, and others.
PM emissions may be composed of black smoke (soot), sulfates generated by sulfur in fuel, and components of unburned fuel and oil. Although the mass of PM emissions emitted from spark-ignited engines is relatively low as compared to diesel engines, the mean diameter of the PM is significantly smaller. It is generally believed that smaller diameter PM is more harmful to health as it can penetrate more deeply into the lungs. Therefore, the control of PM emissions from spark-ignited engines is important, in addition to the existing control of hydrocarbon (HC), carbon monoxide (CO), and oxide of nitrogen (NOx) emissions.